This invention relates to a control system for controlling a PWM-controlled voltage source inverter, and more particularly to an apparatus and method suitable for the PWM control of a voltage source inverter according to a software program using a microcomputer.
As prior art apparatus designated for the PWM control of voltage source inverters, both of those based on a method using an analog circuit and those based on a method using a digital circuit are already known.
A prior art PWM control apparatus based on the former method is disclosed in, for example, a book entitled "New Drive Electronics" written by Naohiko Kaniyama and published by Denki Shoin Publishing Co. in Japan in July, 1986. According to the method described in page 92 of this publication, a voltage command signal (a modulation wave signal) representing the error between a current command signal and an output current of a voltage source inverter is compared in an analog circuit with a given triangular wave signal which is a carrier wave signal, so as to generate a desired pulse pattern used for controlling the PWM-controlled voltage source inverter. In a modification of the former method, the modulation wave signal and the carrier wave signal are compared in a digital fashion. Herein, such a modification is classified to belong to the former method.
A prior art PWM control apparatus for controlling a voltage source inverter according to the latter method using a digital circuit is disclosed in, for example, Japanese Patent Publication No. 55-49509 filed as a patent application by Mitsubishi Electric Co., Ltd. in Japan on December 28, 1973 and published on December 12, 1980. The cited publication proposes a method which attempts to generate an improved rectangular output from the voltage source inverter by alternately applying on-off control to two phases and generally increasing and decreasing the pulse width of the rectangular wave output.
On the other hand, a method for controlling a current source inverter instead of a voltage source inverter is disclosed in JP-A-62-163577 filed as a patent application by Hitachi, Ltd. on January 11, 1986 and laid open on July 20, 1987. The disclosure of the cited publication proposes to generate a sine wave output from a PWM-controlled current source inverter by converting instantaneous values of a sine wave into corresponding pulse widths. Further, U.S. Pat. No. 4,870,556 and whose assigners include some of the inventors of the present application, discloses a method for controlling a current source inverter by a microcomputer.
In the related art described above, any consideration is not especially given to application of the PWM control apparatus for the control of the voltage source inverter according to a software program, and various problems such as a capability of generation of an output waveform sufficiently close to a sinusoidal waveform and a capability of reduction of the size of the PWM control apparatus remain still to be solved.
More specifically, in the case of the PWM control apparatus based on the method using the analog circuit, a plurality of comparators each including an operational amplifier are essentially required, and this requirement leads to problems such as a lot of time and labor consumed for adjustment of the comparators and an inevitable increase in the circuit scale. Also, in the case of the PWM control apparatus based on the method using the digital circuit, generation of an ideal sine wave output is difficult because the two phases only are on-off controlled. Further, because the control circuit is composed of discrete circuit parts, there is yet considerable room for improvement from the viewpoint of simplification of the circuit scale.
On the other hand, although the methods disclosed in JP-A-62-163577 and U.S. Pat. Application Ser. No. 2283 cited above are satisfactory in that the PWM signal has a pulse pattern close to the waveform of a sine wave, and that the structure of the control circuit is simplified, the applicable subject of those methods is limited to the current source inverter, and the methods cannot be directly applied to the voltage source inverter.